Communication systems almost always have a goal of efficiently using the electromagnetic spectrum allocated to them. Several methods to achieve this goal have been developed, including Frequency Division Multiple Access (FDMA) , Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA). In FDMA systems, each earth station has an assigned bandwidth between specified frequencies. The total satellite capacity is usually divided into several transponders. This bandwidth can be further divided, and parts assigned to specific customers and their earth stations. If there are n carriers being sent through the transponder simultaneously, they must be on individual frequencies (f1,f2,f3, . . . ,fn) with non-overlapping bandwidths separated by guard bands. Thus, each assigner has exclusive use of its assigned bandwidth.
In TDMA systems there are periodically recurring time slots during which message information of a particular user is transmitted/received. The users are assigned to particular time slots controlled by a master controller synchronized by a master clock. Each discrete channel set can be assigned one time slot. Each coverage area can use the same frequency channel or channel set without interference because the users in each coverage area only receive or transmit information during their assigned time slot. Each time slot can contain one message packet (i.e., single message time slots) or can contain multiple message packets (i.e., multiple sub-time slots each containing a single message). Even further, methods have been developed for dynamically assigning time slots based on the spectral capacity of each of the coverage areas. Such a method is disclosed in U.S. Pat. No. 5,604,733 issued to Hargrave.
CDMA is a spread spectrum technique where specified members of a set of orthogonal or nearly orthogonal spread spectrum codes are allocated, each using the full channel bandwidth. Two common spread spectrum techniques are direct-sequence and frequency hopping. These communication techniques are well known in the art.
An emerging goal of communications systems is to provide flexibility in service to its users. Some satellite communications systems offer a variable data rate to its users. This option, however, is typically only available when a drop-out occurs in the primary data rate communications.
Thus, there exists a need for a satellite-based communications system that provides efficient use of the allocated electromagnetic spectrum while offering flexibility in services to its users, such as variable data rate and location based on the users' needs.